1. Field of the Invention
This invention relates generally to well cementing compositions and methods and, specifically, to a dry admix for such compositions for achieving matrix control and zonal isolation through improved bonding, fluid loss control, reduced stratification of solids, decreased permeability, and increased filtrate viscosity.
2. Description of the Prior Art
Cement compositions are used in the oil and gas industry to cement the annular space in the well bore between the surrounding formation and pipe or casing. Typically, the cement slurry is pumped down the inside of the casing and back up the outside of the casing through the annular space. The slurry is allowed to set up or harden in the annular space, thereby forming a rigid column which ideally forms a bond with the earth formation as well as with the metal pipe. To achieve a satisfactory primary cementing job, it is important to achieve a tight bond to prevent vertical communication of fluids or gas along or within the column, which could contaminate the producing zone, or permit a loss of reserves.
Adequate fluid loss control is also important in achieving effective primary cementing. Inadequate fluid loss control can result in the formation of a bridge in the annulus opposite a permeable zone, thus isolating a lower zone from the hydrostatic pressure above the bridge. Only a small amount of filtrate loss beneath such a bridge is then necessary to drop the annular pressure to beneath that of the formation pressure. The result is an influx of formation fluids and pressure, creating flow channels and the need for expensive remedial work.
In attempting to control fluid loss from the cementing slurry to the surrounding formation, it is important to reduce the cement matrix permeability and retain water during the initial set, effectively blocking the porous cement structure. One way to do this is to reduce filtrate mobility by increasing the filtrate viscosity to counter the normal thermal thinning of the cement slurry which occurs at down hole temperatures. An increase in filtrate viscosity at down hole temperatures minimizes thermal thinning and increases the retention of the filtrate within the cement matrix. Conventional fluid loss polymers do not effectively counteract thermal thinning with increased temperature thereby allowing fluid loss to the formation and promoting stratification of solids within the cement column.
In order to lessen the loss of fluid from the aqueous cement slurry, various materials have been employed in the past. At lower temperatures, cellulosic materials such as methyl cellulose, carboxymethyl cellulose, hydroxymethyl cellulose, and the like, have proved to be effective. As temperatures in the well bore increase with depth, other types of synthetic polymers have been utilized. The use of polyamine compounds is taught, for instance, in U.S. Pat. No. 3,491,049, to Gibson et al, issued Jan. 20, 1970. However, polyamines do not generally function as fluid loss additives when used alone but must be used with a companion material, typically one of the so-called "dispersant" materials. The Gibson et al patent teaches the use of a sulfonated naphthalene condensate dispersant as an additional additive to the cement slurry which cooperates with the polyamine additive to provide satisfactory fluid loss in cement slurries used at about 200.degree. F. and below. The sulfonated naphthalene dispersant is typically a low molecular weight material, e.g., in the range from about 1,000 to 3,000.
Sulfonated polystyrene has also been used in the past as a fluid loss additive for cement compositions. U.S. Pat. No. 3,234,154 to Martin, issued Feb. 8, 1966, teaches the use of a sulfonated polystyrene polymer having a molecular weight in the range of 300,000 to 4,000,000. The sulfonated polystyrene is used in conjunction with a lignin amine derivative.
U.S. Pat. No. 3,409,070 to Harrison, issued Nov. 5, 1968, mentions that polyvinyl alcohol and polyvinyl acetate can be used as polymeric fluid loss agents in oil well cements. The particular type of polyvinyl alcohol utilized is not stressed and the example given in the patent uses a polyvinyl alcohol which is 88% hydrolyzed. It is also applicant's understanding that the polyvinyl alcohol polymers used as fluid loss agents in the past were always soluble in the cement slurry at ambient temperatures.